In electro-optical distance measurement, in the distance range between 100 m and several km, the measurement is decisively influenced by the refractive index of air. The propagation velocity of an optical pulse emitted in the electro-optical distance measurement or of a signal path modulated in any desired manner is determined by the group refractive index n. This applies both to electro-optical distance-measuring instruments which are based on the phase principle and to those based on the transit time measurement principle.
Refractive index and group refractive index are not constant quantities but depend predominantly on wavelength, frequency, temperature, atmospheric pressure, gas mixture, and moisture content of the atmosphere prevailing in each case.
In virtually all devices for electronic distance measurement (EDM devices), the effect of the atmospheric parameters is added as a distance correction in a further computational step after completion of the actual distance measurement. The critical atmospheric parameters are measured in each case not with the distance-measuring instrument but with other, separate instruments, such as thermometer, barometer and hygrometer.
The distance D0 (raw measurement) directly measured and displayed on the electronic distance-measuring instrument (EDM) relates to a specific group refractive index n. On the basis of the additionally measured meteorological parameters comprising temperature T, atmospheric pressure p and relative humidity RH, the true group refractive index n=n(T,p,RH, . . . ) can be calculated. By means of a so-called atmospheric correction
      Δ    ⁢                  ⁢    D    =            D      0        ·          (                                    n            0                    -          n                n            )      
the true distance D can be determined:D=D0+ΔD 
By means of this atmospheric “post-processing” method, as a rule accuracies of distance measurement in a region of 1 ppm are achieved. On the other hand, if temperature T and atmospheric pressure p are not known or are not representative over the entire optical path, the measured raw distance D0 can easily deviate by 30 ppm or more from the true value.
In the case of longer distances, which moreover generally pass over irregular topography, a reliable determination of the effective group refractive index from meteorological data is problematic at the end points of the distances. Attempts to determine these data along the target beam have not been successful to date.
One of the basic concepts is the utilization of the spectral broad-band dispersion by measuring the distance with light or electromagnetic radiation of two different wavelengths. The 2- or multi-color method of measurement has been known since about 1975. In the case of simultaneous distance measurement using at least 2 different electromagnetic wavelengths, in the optical or microwave range, atmospheric disturbance parameters can be approximately determined by means of the spectral broad-band dispersive behavior of the atmosphere.
Corresponding theories are based on the spectral broad-band formulae of Edlen and Barrel & Sears. (Ref. Rainer Joeckel, Manfred Stober: Elektronische Entfernungs- und Richtungsmessung [Electronic distance and direction measurement], Verlag Konrad Wittwer.)
The results of the distance measurement of the 2 carrier wavelengths are Dr and Db, and the corresponding refractive indices are nr and nb. The true distance is obtained by the following formula for the distance correction:
  D  =            D      r        -                  (                              D            b                    -                      D            r                          )            ·              (                                            n              r                        -            1                                              n              b                        -                          n              r                                      )            
The actual problem of the 2-color method based on the model of the spectral broad-band formula consists in the accuracy of resolution with which the difference between the distances (Db−Dr) has to be determined. The further apart the two carrier wavelengths are, the smaller and more advantageous is the model parameter
  Q  =      (                            n          r                -        1                    (                              n            b                    -                      n            r                          )              )  
Since the accuracy of resolution is independent of the distance, these types of two-color instruments are potentially superior to the one-color measurement only in the case of relatively long distances substantially above 2 km.
Known 2-color instruments are, for example, Goran I from the National Physical Laboratory (Teddington, UK) with λb=458 nm and λg=514 nm, and the large Q=57. For a distance error of 1 mm, the required accuracy of resolution is 0.02 mm. Since the latter can be realized only with very great inconvenience, if at all, this method has not become established to date.
U.S. Pat. No. 5,233,176 discusses a device using the 2-color method, which compensates the atmospheric effects on measurement by evaluating the deviation of two laser beams of different wavelengths from a respective reference beam path. Here, the laser light is emitted at two different carrier wavelengths in short pulses. The dispersive effect is deduced from the dispersive shift of the two beam paths from the straight line, and the measurement is corrected.
A considerable disadvantage of all devices to date which use 2 or 3 carrier wavelengths is the utilization of the slightly variable broad-band optical dispersion. The procedure is often based on the broad-band models of Barrel & Sears or on the formulae according to Edlen. Furthermore, only broad-band methods have been used to date in the microwave range. The main disadvantage of traditional broad-band techniques is that the distance correction obtained is often below the quality of the classical atmospheric correction based on the determination of the meteorological parameters T, p, and RH.